Extracellular matrix (ECM)-degrading proteinases of the matrix metalloproteinase (MMP) gene family have been implicated in the pathogenesis of tumor progression, yet also play a role in normal tissue remodeling. This project will examine at the level of molecular regulation how MMPs alter the microenvironment of mammary gland during development and remodeling. The hypothesis being tested is that the molecular targets of MMPs are essential in the interaction between mammary epithelial cells and their mesenchymal stroma. In the previous grant period, using transgenic mice and mammary cultures, the investigators showed that a critical balance exists between MMPs expressed by stromal cells and mammary epithelial cells to regulate lobuloalveolar development, lactational differentiation and involution, and that misregulation of an MMP results in inappropopriate development, and in neoplastic and malignant alterations. The present proposal addresses the mechanisms by which these MMPs orchestrate the cross-talk between mesenchymal and epithelial compartments. The approach uses parallel in vivo and culture experiments to analyze mammary gland development and dysfunction. Ductal and alveolar mammary epithelial cells and mammary fibroblasts with genetically altered MMP or inhibitor levels will be studied in culture, and in normal or transgenic mice, and analyzed for growth, morphogenesis, differentiation, and invasive behavior. The mechanisms by which MMPs stimulate ductal proliferation, branching and lobuloalveolar development will be compared and contrasted with the mechanisms by which MMPs induce involution and apoptosis in mammary alveolar epithelia cells. In both cases the contribution of mammary stromal cells will be evaluated. The MMPs that mediate these two distinct sets of processes and their molecular targets in the extracellular matrix or on the cell surface will be elucidated. The direct effects of MMPs on the phenotype of epithelial cells and fibroblasts will be studied to evaluate their roles in altering proliferation, inducing fibrosis, invasion and tumorigenesis. These approaches will elucidate how MMPs alter the cellular microenvironment, and what the critical responses are in normal and abnormal mammary gland epithelium and stroma. These studies address an important aspect of women s health, that of how MMPs regulate normal and neoplastic mammary gland function. These studies may form the basis of intervention and therapy in breast cancer, not only in the late malignant stages, but potentially in the premalignant lesions.